Arthroplasty is surgery to relieve pain and restore range of motion by realigning or reconstructing a joint. Typical arthroplastic options include joint resection, interpositional reconstruction, and total joint replacement. Joint resection involves removing a portion of a bone from a joint to create a gap between the bone and the corresponding socket, thereby improving the range of motion. Scar tissue eventually fills the gap. Pain may be relieved and motion restored, but the joint is typically less stable. Interpositional reconstruction reshapes the joint and adds a prosthetic disk between the bones forming the joint. The prosthesis can be made of plastic and metal or from body tissue such as fascia and skin. If the joint does not respond to the more conservative treatments (which may include medication, weight loss, activity restriction, and/or use of walking aids such as a cane), joint replacement is often considered appropriate. Joint replacement (i.e., total joint arthroplasty) is the surgical replacement of a joint with a prosthesis. Many joint replacements are needed because arthritis has caused the joint to stiffen and become painful to the point where normal daily activities are no longer possible. 1See Gale Encyclopedia of Medicine, Gale Research (1999), found at http://www.findarticles.com/cf_dls/g2601/0007/2601000783/p1/article.jhtml?term=; see also the YourSurgery.com® website, found at http://www.yoursurgery.com/ProcedureDetails.cfm?BR=5&Proc=30. 
Arthroplasty, especially joint replacement, is becoming an increasingly prevalent treatment. For example, it has been reported that more than 170,000 hip replacements and more than 200,000 knee replacements are performed in the United States each year. A knee prosthesis has three main components, a femoral implant, a tibial implant, and an disk-like insert or cushion. The femoral and tibial implants cap the ends of the distal femur and the proximal tibia, respectively. They are typically made of metal and include posts for driving them into the femur and tibia, respectively. The cushion is typically made of a strong, smooth, low-wearing plastic.
In a typical knee replacement operation, the surgeon makes an anterior incision spanning over the distal femur, the knee, and the proximal tibia, and then separates the femur and the tibia from the surrounding tissues. Next, the surgeon secures a resection guide to the proximal end of the tibia. A resection guide is a jig or template configured to provide a desired cutting angle for a saw blade or other resection tool. Conventional resection guides are used somewhat similarly to the manner in which a carpenter uses a miter box to achieve a desired angle for cutting wood. The surgeon uses the resection guide to position a saw blade or other suitable resection tool and cuts off the tibial plateau (i.e., the upper end of the tibia which forms the lower part of the knee joint). This prepares the tibia to receive the tibial implant (which will form an artificial tibial plateau). To determine the longitudinal axis of the femur, the surgeon inserts an intramedullary (“IM”) rod through a hole near the center of the joint surface of the lower end of the femur and into the medullary (i.e., bone marrow) canal that runs longitudinally in the center of the femur. Then, the surgeon aligns one or more additional resection guides for cutting the distal femur as required for receiving the femoral implant (which will form the upper part of the artificial knee). Typically, the surgeon aligns these resection guides relative to the angle of the artificial tibial plateau and the longitudinal axis of the femur. Finally, the surgeon drives the posts of the implants longitudinally into the distal femur and proximal tibia, respectively, cements them in place, secures the cushion to the top of the tibial implant, and closes the incision.
In general, artificial knees are designed to mimic the operation of natural knees. A healthy, natural knee is not merely a simple hinged joint that bends backward (flexion). It also has a rotary motion that locks the femoral condyles into the tibial plateau on straightening (extension) of the leg. On extension of the knee, the ligaments become tight and convert the knee into a rigid locked structure. The knee unlocks on flexion, allowing an increased range of motion as the lower leg swings backward. In operation of a conventional artificial knee, the lower surface of the femoral implant glides on the upper surface of the cushion (which stays sandwiched between the femoral implant and the tibial implant).
But complications may result if the distal femur is not resected properly (i.e., if the surgeon does not cut the distal femur at proper angles relative to the artificial tibial plateau and the longitudinal axis of the femur). Such complications can include increased wear of the plastic surfaces of the prosthesis; bending, cracking or fracture of the bones; dislocation, excessive rotation or loss of motion of the prosthesis; and/or angular deformity of the joint. Naturally, proper resection requires proper alignment of the resection guide(s).